CVD condeposition of A1 and one or more reactive (gettering) elements to form protective aluminide coating

ABSTRACT

CVD aluminide coatings including a small concentration of a reactive, gettering element for surface active impurities dispersed therein are formed for improved oxidation resistance. The aluminide coatings are formed by CVD codeposition of Al and the gettering element on the substrate using coating gases for the gettering element generated either outside or inside the coating retort depending on the chlorination temperature needed for the particular gettering element.

FIELD OF THE INVENTION

[0001] The present invention relates to the chemical vapor codepositionof multiple elements and particularly of aluminum and one or more active(gettering) elements, such as Hf, Zr, Y, to form a protective aluminidecoating bearing the active (gettering) element on a substrate ineffective amount to improve coating oxidation/corrosion resistance.

BACKGROUND OF THE INVENTION

[0002] It is well known to improve the oxidation and/or corrosionresistance of various nickel, cobalt and/or iron base superalloy gasturbine engine components by forming a protective aluminide diffusioncoating thereon. One coating process used to form aluminide coatings onsuperalloy substrates involves a pack process (pack cementation) whereinthe substrate to be coated is placed in a particulate bed of coatingmaterial (e.g. aluminum particulates, halide activator, and inertdiluent) and heated in a retort to an elevated coating temperature togenerate an aluminum halide coating gas that reacts with one or moresubstrate elements (e.g. Ni for a nickel base superalloy substrate) toform the aluminide coating (e.g. nickel aluminide) on the substrate.U.S. Pat. Nos. 3,544,348, 3,961,098, 4,070,507, and 4,132,816 disclosepack coating processes for forming aluminide coatings.

[0003] Another coating process used to form aluminide coatings onsuperalloy substrates involves a chemical vapor deposition process (CVDprocess) wherein the substrate to be coated is placed in a reactor,heated to elevated coating temperature, and a coating gas comprisingaluminum halide (e.g. Al trichloride or subchlorides) and a carrier gas,such as hydrogen, is introduced to the reactor to deposit aluminum onthe substrate for reaction therewith to form the aluminide coatingthereon. The Gauje U.S. Pat. No. 3,486,927 represents an early effortdirected to a CVD coating process.

[0004] The CVD process can be used to form aluminide coatings includingadditional alloying elements in the coating. For example, U.S. Pat. No.2,772,985 discloses formation of binary Al—B, Al—Si, Al—Ti, and Al—Zrcoatings on Mo substrates. The patented process first vapor phasedeposits Al on the Mo substrate and then reacts the deposited Al withthe alloying element (B, Si, Ti or Zr) deposited from a second coatinggas. The aluminum halide coating gas and alloying element halide coatinggas are generated in separate parallel generators to provide separatecoating gas streams. The process does not involve codeposition of thecoating elemental constituents on the substrate.

[0005] U.S. Pat. No. 4,687,684 relates to CVD formation of oxidationresistant Si and Cr-enriched aluminide diffusion coatings on asuperalloy substrate. The patent provides for sequential introduction ofthe aluminum halide coating gas and the silicon or chromium halidecoating gas to the coating reactor. The patent does not involvecodeposition of the coating elemental constituents on the substrate.

[0006] U.S. Pat. No. 5,015,502 discloses a CVD coating process forforming aluminide coatings containing a gettering element (e.g. Y) bychlorination of a source of aluminum containing Y and one or more of Si,Cr, Co, etc. A preferred source is described as an Al—Y—Si alloy.

[0007] Aluminum and silicon have been intermittently codeposited on aheated superalloy substrate by CVD such that a silicon-modifiedaluminide coating is formed. In particular, low silicon contents oftrace to 8 weight % are desirable to improve the oxidation/corrosionresistance of the aluminide coating. Since typical silicon precursors,such as silicon tetrachloride and dichlorosilane, are much less stablethan aluminum trichloride, a short pulse (e.g. 7.5 minutes) of thesilicon halide or silane coating gas is provided intermittently (e.g.once every two hours) during the continuous CVD aluminum deposition toobtain the desired silicon concentration in the coating. Since nointermetallic compounds form between aluminum and silicon and thecoating is applied at high temperature, silicon can be deposited on thesubstrate from pulses of silicon tetrachloride or dichlorosilane andthen it diffuses during the aluminizing cycle to give a homogenoussilicon modified aluminide coating.

[0008] If Al and a reactive element (such as Hf, Zr and Y) which formstable intermetallic compounds with Si are continuously codepositedwhile Si is intermittently codeposited by the aforementioned CVD pulsingtechnique, then the resulting modified aluminide coating will beheterogenous in that the pulses of silicon coating gas produce layers inthe coating that contain a large volume fraction of silicon richintermetallic compounds. Such a coating structure wherein the majorityof the reactive element (gettering/active element) exist asintermetallic compound layers does not exhibit the superior oxidationresistance of a modified aluminide coating wherein the reactive elementis uniformly dispersed throughout the aluminide coating.

[0009] Thus, there is a need for the continuous deposition of aluminumand a reactive element, such as Hf, Zr, and/or Y, with or without Si toobtain a more homogenous coating for oxidation resistance.

[0010] It is an object of the invention to provide a CVD method andapparatus for codepositing multiple elements on a substrate in a mannerto overcome disadvantages discussed above.

[0011] It is another object of the invention to provide a CVD coatingmethod and apparatus for forming aluminide coatings wherein aluminum andone or more reactive elements for surface active superalloy substrateimpurities (e.g., S, P, etc.) are codeposited on a substrate.

[0012] It is still another object of the invention to provide a CVDcoating method and apparatus for forming an aluminide coating includingsilicon and one or more reactive elements for surface active superalloysubstrate impurities wherein aluminum, silicon and the reactiveelement(s) (e.g. Hf, Zr, Y) are codeposited on the substrate.

[0013] It is still a further object of the invention to provide a coatedsuperalloy article having an aluminide coating thereon including one ormore reactive elements for surface active superalloy substrateimpurities uniformly distributed in a region of the coating orthroughout the coating for improved oxidation resistance.

SUMMARY OF THE INVENTION

[0014] The present invention provides in one embodiment method/apparatusfor forming on a substrate a chemical vapor deposited (CVD'ed) aluminidecoating including one or more reactive elements, such as Hf, Zr, and Y,dispersed therein by virtue of codeposition of Al and the reactiveelement(s) during CVD coating. In a particular embodiment of theinvention wherein the reactive element, such as Hf and Zr, can behalogenated (e.g. chlorinated) at relatively low temperatures (e.g. lessthan 600° C.), the invention comprises flowing a first halide precursorgas in a carrier gas in contact with a first source comprising aluminumdisposed outside a coating retort to generate an aluminum halide firstcoating gas, flowing a second halide precursor gas in an inert carriergas in contact with a second source comprising a reactive element (e.g.Hf or Zr) disposed outside the coating retort to generate a secondhalide coating gas of the reactive element, and introducing the firstand second coating gases concurrently into a coating retort in which thesubstrate at elevated temperature is disposed to codeposit Al and thereactive element on the substrate for coating formation.

[0015] The first coating gas preferably is formed by flowing hydrogenchloride in a hydrogen carrier gas in contact with Al particulatesheated to a reaction temperature to form aluminum trichloride. Thesecond coating gas preferably is formed by flowing hydrogen chloride inan inert carrier gas in contact with particulates comprising an areactive element selected from the group consisting essentially of Hfand Zr heated to a reaction temperature to form the tetrachloridethereof.

[0016] In another particular embodiment of the invention wherein thereactive element, such as Y, can be halogenated (e.g. chlorinated) onlyat relatively high temperatures (e.g. greater than 600° C.), theinvention comprises flowing a halide precursor gas in a carrier gas incontact with a first source comprising aluminum disposed outside acoating retort to generate an aluminum halide coating gas, flowing thefirst coating gas into the coating retort in contact with a secondsource comprising a reactive element (e.g. Y) disposed inside the retortand heated to the necessary reaction temperature to convert a portion ofthe first coating gas to a halide coating gas of the reactive element,and contacting the coating gases concurrently with the substrate atelevated temperature in the retort to codeposit Al and the reactiveelement on the substrate for coating formation.

[0017] In this embodiment of the invention, prior to contacting thesubstrate, the unconverted portion of the first coating gas is flowed incontact with a secondary source comprising an aluminum alloy in thecoating retort to increase the activity of aluminum therein. Forexample, the first coating gas can comprise aluminum trichloride in ahydrogen/inert carrier gas. The activity of aluminum in the unconvertedportion of the first coating gas is increased by forming aluminumsubchlorides by contact with a secondary source comprising an aluminumalloy (e.g. Al—Co or Al—Cr) in the retort downstream of the getteringelement source. This provides the desired amount of Al deposition on thesubstrate.

[0018] In this embodiment of the invention, the first coating gas caninclude a tetrachloride of the reactive element (such as hafnium orzirconium tetrachloride) generated outside the coating retort so as tocodeposit two reactive elements on the substrate.

[0019] Also in this embodiment of the invention, a silicon halidecoating gas can be introduced into the coating retort in a manner toby-pass the reactive element source so as to concurrently contact thesubstrate with the other coating gases present to codeposit Al, Si, andthe reactive element(s) on the substrate for coating formation.

[0020] The present invention provides in a further embodimentmethod/apparatus for codepositing first and second elements on asubstrate. This embodiment involves flowing a halide precursor gas in acarrier gas in contact with a first source comprising a first elementdisposed outside a coating retort to generate a first halide coatinggas, flowing the first coating gas into the coating retort in contactwith a second source comprising a second element disposed inside thecoating retort to convert a portion of the first coating gas to a halidecoating gas of the second element, and contacting the first and secondcoating gases concurrently with the substrate at elevated temperature inthe coating retort to codeposit the first and second elements on thesubstrate.

[0021] The present invention provides a coated substrate comprising asubstrate and a CVD'ed aluminide diffusion coating thereon having adispersion of one or more reactive elements (e.g. Hf, Zr, Y) in a region(e.g. inner coating region or outer coating region) or throughout theentire coating by virtue of the reactive element(s) being codepositedwith aluminum on the substrate. The aluminide coating also may includeSi uniformly distributed therein by codepositon with Al and the reactiveelement(s).

[0022] The aforementioned objects and advantages of the presentinvention will become more readily apparent from the following detaileddescription of the invention taken with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic view of CVD coating apparatus for forming analuminide coating including a reactive element, such as for example Hfor Zr, for surface active substrate impurities using coating gasesgenerated by chlorination at a relatively low temperature outside theretort in accordance with one embodiment of the invention.

[0024]FIG. 2 is a schematic view of a CVD coating apparatus for formingan aluminide coating including Si in addition to the reactive element inaccordance with another embodiment of the invention.

[0025]FIG. 3 is a schematic view of CVD coating apparatus for forming analuminide coating including a reactive element using a coating gas forthe gettering element generated by chlorination at higher temperaturesinside the retort in accordance with still another embodiment of theinvention.

[0026]FIG. 4 is a schematic view of CVD coating apparatus for forming analuminide coating including a reactive element using a coating gas forthe active/gettering element generated by chlorination at highertemperatures inside the retort in accordance with an alternativeembodiment of the invention.

[0027]FIG. 5 is a schematic view of CVD coating apparatus similar tothat of FIG. 3 for forming an aluminide coating including reactiveelement (e.g. Hf or Zr) using a coating gas generated by chlorinationoutside the retort and a second reactive element (e.g. Y) using acoating gas generated by chlorination at higher temperatures inside theretort in accordance with still an additional embodiment of theinvention.

[0028]FIG. 6 is a schematic view of CVD coating apparatus similar tothat of FIG. 4 for forming an aluminide coating including silicon and areactive element (e.g. Hf) using coating gases generated by chlorinationoutside the retort and a second reactive element (e.g. Y) using acoating gas generated by chlorination at higher temperatures inside theretort in accordance with still an additional embodiment of theinvention.

[0029]FIG. 7 is a photomicrograph at 1000× of a CVD aluminide coating inaccordance with one embodiment of the invention including a uniformdispersion of hafnium-rich particles therein.

DETAILED DESCRIPTION

[0030] FIGS. 1-6 illustrate CVD coating apparatus in accordance withillustrative embodiments of the invention for forming a CVD aluminidecoating including an effective concentration of one or more reactiveelements (gettering/active elements) for surface active substrateimpurities (e.g. S, P, etc.) dispersed uniformly at a region orthroughout the coating to improve oxidation/corrosion (sulfidation)resistance. By reactive element for surface active substrate impuritiesis meant a coating alloying component that readily combines with surfaceactive elements (such as S, P, etc.) from the substrate to tie up theseelements as a compound and prevent migration thereof to thecoating/outer oxide interface where surface active substrate impuritiesare known to reduce coating life in elevated temperature oxidation andcorrosion tests. Known reactive (gettering/active) elements usedheretofore in aluminide coatings to this end are selected from the groupconsisting essentially of Hf, Zr and Y.

[0031] The present invention provides a method and apparatus for CVDcodeposition on a substrate to be protectively coated of Al and one ormore of reactive elements for surface active substrate impurities in amanner that the reactive element(s) is/are distributed uniformly at aparticular region (e.g. inner coating region proximate the substrate orouter coating region remote from the substrate) or throughout thecoating thickness without layering so as to improve coatingoxidation/corrosion resistance. FIGS. 1-6 are offered for purposes ofillustrating, but not limiting, the invention to this end.

[0032]FIG. 1 illustrates one embodiment of the invention wherein the CVDcoating gases for codepositing Al and a reactive element are generatedat relatively low halogenation (e.g. chlorination) temperatures outsideof the coating reactor or retort R disposed within furnace F. CVDaluminide coatings generally comprising Al—Hf, Al—Hf—Si, Al—Zr,Al—Zr—Si, and Al—Hf—Si—Zr can be formed by this embodiment on substratessince the aluminum trichloride and tetrachloride of Hf, Si, and/or Zrcan be formed at temperatures less than 600° C. achievable outside thecoating retort R.

[0033] For purposes of illustration and not limitation, the coatingretort R of FIG. 1 is illustrated as receiving two coating gas streamsS1 and S2. Stream S1 comprises aluminum trichloride in a hydrogencarrier gas for depositing Al on the substrates S disposed in the retortR. Stream S2 comprises hafnium tetrachloride in a He (or other inertgas) carrier gas for depositing Hf on the substrates S concurrently withthe Al from stream S1.

[0034] To this end, the CVD coating apparatus is shown comprising acoating gas generator housing 12 having a reaction chamber 12 a with aninlet 14 and outlet 16. Disposed in the reaction chamber 12 a is asource of aluminum, such a bed B1 of aluminum particulates (Alcoa GrannyP pellets). The generator housing 12 is heated by electric resistanceheating to provide a desired reaction temperature (e.g. 290° C.) thereinto effect the desired reaction between the aluminum source and theprecursor gas stream SP1. In the illustrative embodiment of FIG. 1, theprecursor halide gas stream SP1 comprises ultra high purity hydrogencarrier gas and hydrogen chloride gas supplied from respectiveconventional bulk supply (H₂) and cylinder (HCl) (not shown) to a commonsupply conduit 20 where they are mixed uniformly. The hydrogen chloridetypically comprises about 1 to about 20 volume % of the precursor gasstream SP1, the balance being hydrogen. The ratio of HCl/hydrogen andflow rate of the precursor gas stream SP1 is adjusted to providecomplete conversion of the hydrogen chloride to AlCl₃ at 290° C. in thehydrogen carrier gas. Typically, the AlCl₃ gas comprises 1 to 10 volume% of the coating gas stream S1. Stream S1 is introduced into the retortR via inlet I so as to contact the substrates S disposed therein.

[0035] The coating apparatus also is shown comprising a coating gasgenerator housing 32 having a reaction chamber 32 a with an inlet 34 andoutlet 36. Disposed in the reaction chamber 32 a is a source of hafnium,such a bed B2 of hafnium particulates (pellets) having a size range ofgreater than ¼ inch to less than ⅝ inch. The generator housing 32 isheated by electric resistance heating to provide a desired reactiontemperature (e.g. 430° C.) therein to effect the desired reactionbetween the hafnium source and the precursor gas stream SP2. In theillustrative embodiment of FIG. 1, the precursor halide gas stream SP2comprises ultra high purity helium (or other inert gas) carrier gas andhydrogen chloride gas supplied from respective conventional bulk supply(inert) and cylinder (HCl) (not shown) to a common supply conduit 40where they are mixed uniformly. The hydrogen chloride typicallycomprises about 4 to about 70 volume % of the precursor gas stream SP2,the balance being inert gas. The ratio of HCl/inert gas and flow rate ofthe precursor gas stream SP2 is adjusted to provide complete conversionof the hydrogen chloride to HfCl₄ at 430° C. in the inert carrier gas.Typically, the HfCl₄ gas comprises 1-5 volume % of the coating gasstream S2. The carrier gas for stream S2 comprises an inert gas since Hf(and other of the aforementioned gettering elements) form hydrides byexothermic reactions. The inert carrier gas avoids formation of suchhydrides.

[0036] Coating gas stream S2 is introduced into the retort R via inlet Iconcurrently with stream S1 so as to contact the substrates S disposedtherein and heated to an elevated coating temperature, whereby Al and Hfare codeposited on the substrates S and form an aluminide diffusioncoating including Hf uniformly dispersed therein by virtue of the Al andHf being codeposited. Flow of the coating gas streams S1, S2 can becontrolled as desired to form a tailored coating wherein the reactiveelement is uniformly dispersed at an inner coating region, outer coatingregion, or throughout the central coating thickness. The substrates areheated to an elevated coating temperature, such as about 1100° C. for anickel base superalloy. Spent coating gases are removed from the retortvia exhaust E.

[0037] By adjusting the partial pressure of the HfCl₄ in the CVD retortR, the relative concentrations of Al and Hf in the CVD coating can bevaried between, for example, 25 to 30 weight Al and trace to 5.0 weight% Hf. For example, aluminide coatings including 0.2, 0.427, 1.2, 1.9,and 2.51 weight % Hf (as determined by electron microprobe analysis ofthe CVD coatings) uniformly dispersed therein have been formed on PWA1480, PWA 1484, Rene 80, Rene 41 and IN738 nickel base superalloysubstrates.

[0038] Coating times at substrate temperatures of 1080° C. of 10 to 20hours were used. The streams S1, S2 were flowed axially past thesubstrates in the manner illustrated in FIG. 1 in some coating trialsand also radially past the substrates in the manner illustrated in U.S.Pat. No. 5,264,245 in other coating trails.

[0039] The invention was effective to codeposit Al and Hf on thesubstrates in a manner to form a CVD aluminide diffusion coating havinga uniformly distributed concentration of Hf throughout the coating forimproved cyclic oxidation resistance compared to a simple aluminidecoating without Hf therein (e.g. 2 to 7 times better cyclic oxidationresistance) as determined by cylic oxidation tests. A one hour testingcycle included 50 minutes at 1100° or 1177° C. and 10 minutes cooling toroom temperature. Specimen weight change was measured after every 50such cycles.

[0040] Similarly, Al and Zr can be codeposited on substrates to form aCVD aluminide diffusion coating having a uniformly distributedconcentration of Zr at a particular coating region or throughout thecoating for improved cyclic oxidation resistance (e.g. 2 to 5 timesbetter) than a simple aluminide coating. For example, a bed of Zrpellets having a size range of greater than ¼ inch to less than ⅝ inchcan be disposed in the generator 32 in lieu of the Hf pellets. Thegenerator housing 32 is heated a reaction temperature of 430° C. toeffect the desired reaction between the Zr source and the precursor gasstream SP2, which comprises ultra high purity helium (or other inertgas) carrier gas and hydrogen chloride gas supplied from respective bulksupply (He) and cylinder (HCl1) (not shown) to the common supply conduit40 where they are mixed uniformly. The ratio of HCl/He and flow rate ofthe precursor gas stream SP2 are adjusted to provide complete conversionof the hydrogen chloride to ZrCl₄ at 430° C. in the He carrier gas.Typically, the ZrCl₄ gas comprises 1 to 5 volume % of the coating gasstream S2. The carrier gas for gas stream SP2 and thus S2 comprises aninert gas since Zr also forms hydrides by exothermic reactions.

[0041] The coating gas streams S1 (AlCl₃/hydrogen ) and S2 (ZrCl₄/He)are introduced into the retort R via inlet I to contact theaforementioned heated substrates S disposed, whereby Al and Zr arecodeposited on the substrates S and form an aluminide diffusion coatingincluding Zr uniformly dispersed therein by virtue of the Al and Zrbeing codeposited.

[0042] By adjusting the partial pressures of the ZrCl₄ in the CVD retortR, the relative concentrations of Al and Zr in the CVD coating can bevaried. For example, aluminide coatings including 0.093; 0.14; 0.17;0.20; 0.399; 0.473 and 0.874 weight % Zr (as determined electronmicroprobe analysis of the CVD coatings) uniformly distributed thereinhave been formed on nickel base superalloy substrates.

[0043]FIG. 2 illustrates the coating retort R as receiving two coatinggas streams S1′ an S2′ wherein stream S1′ comprises aluminum trichlorideand silicon tetrachloride in a hydrogen carrier gas for codepositing Aland silicon on the substrates S disposed in the retort R. Stream S2′corresponds to stream S2 described above for depositing Hf on thesubstrates S. Thus, the apparatus of FIG. 2 is effective to codepositAl, Si and Hf on the substrates S.

[0044] The coating apparatus of FIG. 2 differs from that of FIG. 1 inusing a coating gas generator 12′ in which a bed B1′ of Al pellets (sizerange greater than ¼ inch to less than ½ inch) and bed B1′′ of Sipellets (size range greater than ¼ inch to less than 1 inch) aresequentially disposed (Si pellets downstream from the Al pellets) andthe flow rate of the precursor gas stream SP1′ is controlled so that theprecursor halide gas stream first contacts the Al pellets to form arelatively thermodynamically stable aluminum halide coating gas and thencontacts the Si pellets to form a less thermodynamically stable siliconhalide coating gas.

[0045] The generator housing 12′ is heated to provide a desired reactiontemperature (e.g. 290° C.) to effect the desired reaction between thealuminum pellets (Al source) and the silicon pellets (Si source) withthe precursor halide gas stream (HCl/hydrogen). The total flow rate ofthe precursor stream is controlled to leave a minor portion of the HClavailable after the bed B1′ for reaction with the silicon pellets in thereaction chamber to form a small quantity of silicon tetrachloride.Generally, the total flow rate of the precursor halide gas stream iscontrolled to provide a coating gas stream S1′ comprising 1 volume % orless of silicon tetrachloride, 1 to 10 volume % aluminum trichloride andthe balance hydrogen. When such a coating gas stream S1′ contacts aheated substrate in a CVD retort, Al and Si will be codeposited thereonand incorporated into the aluminide diffusion coating formed thereon.The cogeneration and codepositon of Al and Si in this manner isdescribed further in copending application entitled COGENERATION OF AlAND Si COATING GASES AND CVD CODEPOSITION THEREOF TO FORM Si-BEARINGALUMINIDE COATING (attorney docket no. Howmet 344). The cogeneration andcodeposition of Al and Si in this manner is effective to form aluminidecoatings having up to 9 weight % Si distributed uniformly therein.

[0046] By adjusting the partial pressures of the SiCl₄ and HfCl₄ in theCVD retort R, the relative concentrations of Si and Hf in the CVDcoating can be varied. For example, aluminide diffusion coatingsincluding typically 23 weight % Al with 5.5 weight % Hf and 0.29 weight% Si, 4.3 weight % Hf and 0.3 weight % Si, 0.66 weight % Si and 0.27weight % Si, 0.55 weight % Hf and 0.1 weight % Si, and 0.38 weight Hfand 0.1 weight % Si (as determined by electron microprobe analysis ofthe CVD coatings) uniformly dispersed therein have been formed on PWA1480, PWA 1484, Rene 80, Rene 41 and IN738 nickel base superalloysubstrates after 10 to 20 hours at 1080° C. in the retort R.

[0047] Similarly, Al, Si, and Zr can be codeposited on substrates toform a CVD aluminide coating having a uniformly distributedconcentration of Si and Zr at a particular coating region or throughoutthe coating for improved cyclic oxidation resistance. In particular, thecoating gas stream S2′ will comprise the aforementioned ZrCl₄ in acarrier gas in lieu of the HfCl₄ in carrier gas. CVD aluminide diffusioncoatings including typically 23 weight % Al with 0.15 weight % Zr and0.1 weight % Si and 0.1 weight % Zr and 0.1 weight % Si as determined byelectron microprobe analysis have been formed on the nickel basesuperalloy substrates after 10 to 20 hours at 1080° C. in the retort R.

[0048] Since hafnium and zirconium form stable silicides, it isnecessary to provide continuous deposition on the substrate of siliconto obtain a uniform distribution of hafnium and/or zirconium and siliconin the aluminide coating. The invention provides such continuouscodeposition of hafnium and/or zirconium along with silicon to achieveuniform distribution of hafnium and/or zirconium silicides in thealuminide coating for improved oxidation resistance.

[0049] In addition to the aluminide coatings described above, otheraluminide coatings can be formed in accordance with the aforementionedembodiment of the invention. For example, since Hf and Zr can bechlorinated under the same or similar generator conditions, aluminidecoatings including both Hf and Zr can be formed; e.g. Al—Hf—Zr andAl—Hf—Zr—Si coatings can be formed pursuant to this embodiment of theinvention. In particular, both hafnium and zirconium can be included inthe aluminide coating by generating a coating gas stream including bothHfCl₄ and ZrCl₄ and introducing it to the CVD retort R along with thecoating gas stream S1 or S1′. Such a coating gas stream for Zr and Hfcan be obtained by: chlorinating an appropriate Hf—Zr solid solutionalloy in the generator housing 32, chlorinating the pure metals inseparate external generators, or by cogenerating the metal halides inthe manner described above for Al and Si. The cogeneration technique canbe used when the metal forming the more stable halide (e.g. Hf) ispositioned in bed B1′ and the metal forming the other less stable halide(e.g. Zr) is positioned in bed B1″.

[0050]FIGS. 3 and 4 illustrate coating apparatus in accordance withanother embodiment of the invention for forming a CVD aluminidediffusion coating by codepositing Al and one or more reactive elementsthat are generated at relatively high halogenation (e.g. chlorination)temperatures (e.g. greater than 600° C. maintained inside of the coatingreactor or retort R. CVD aluminide coatings generally comprising Al—Y,Al—Hf—Y, Al—Zr—Y, Al—Si—Y, and Al—Hf—Zr—Si—Y can be formed by thisembodiment on substrates since yttrium tetrachloride can be formed onlyat temperatures greater than 900° C. (e.g. 1080° C.) achievable insidethe coating retort R.

[0051] In particular, in these further embodiments of the invention, thesubstrates S are contacted in the CVD reactor or retort R with a coatingstream S3 generated inside the retort R itself to codeposit Al and agettering element, such as Y, on the heated substrates.

[0052] In the embodiment of the invention illustrated in FIG. 3, theyttrium trichloride coating gas component S3A of coating gas stream S3is generated inside the coating retort R at a location upstream of theinlet I. The yttrium trichloride coating gas stream S3A can be generatedby chlorination at high temperatures of a yttrium bed B4 disposed in theretort R. As a result, the chlorination reaction is effected in the CVDreactor retort R itself.

[0053] That is, the coating gas stream S3A is generated in the retort Rin a separate reaction chamber 92 in which the yttrium bed B4 (yttriumsource) is disposed. The yttrium bed B4 comprises turnings having aparticle size of greater than ¼ inch to less than ¾ inch. Yttriumtrichloride is generated by passing a precursor halide gas stream SP3Acomprising an inert/hydrogen carrier gas (e.g. He/H₂) and aluminumtrichloride through the bed B4 of yttrium particulates maintained at1080° C. in the retort R.

[0054] The precursor gas stream SP3A is generated in a generator housing100 by flowing a 10 to 15 volume % HCl/15 volume % hydrogen/balance Hegas stream SP3 in contact with the heated (290° C.) bed B5 of Al pelletsof the type described above to generate aluminum trichloride in a 15volume % hydrogen/He carrier gas.

[0055] The thermodynamic stability of yttrium trichloride is a functionof the chlorine pressure in the gas stream SP3A such that to achieve thehighest yttrium trichloride concentrations in the retort R, the chlorinepartial pressure should be minimized. Low chlorine partial pressures canbe achieved by passing a mixture of He/H₂ carrier gas and hydrogenchloride through the low temperature aluminum trichloride generator 100operated under conditions to produce complete conversion of the HCl toAlCl₃. Subsequently, the aluminum trichloride and He/H₂ carrier gasmixture enters the CVD reactor or retort R via inlet I where it passesthrough the yttrium bed B4.

[0056] Since aluminum chloride is less thermodynamically stable thanyttrium trichloride, a portion of the stream SP3A undergoes an exchangereaction which generates yttrium trichloride gas and deposits aluminummetal in the bed B4. The appropriate chemical reaction is given below:

AlCl₃ +Y=YCl₃+Al

[0057] Liquid aluminum deposited in the yttrium bed B4 subsequently canreact with the yttrium metal to form yttrium-aluminum intermetalliccompounds. Some of the yttrium-aluminum compounds can melt attemperatures below 1000° C., resulting in melting of the yttriumchloride generator. The portion of the stream SP3A undergoing theexchange reaction is determined by the flow rate of stream SP3A for agiven reaction temperature through bed B4.

[0058] The carrier gas used for stream SP3A is selected to includehelium or other inert gas plus about 15 volume % hydrogen. Thiscomposition of the carrier gas is important and is selected to provide acompromise between two competing processes. Namely, since yttrium formsa pyrophoric hydride by an exothermic reaction, an inert carrier gas isdesirable. However, yttrium hydride inhibits the formation of lowmelting point aluminum-yttrium intermetallic compounds, and consequentlyprevents melting of the yttrium chloride generator.

[0059] Since the conversion of a portion of AlCl₃ in coating gas streamSP3A to YCl₃ lowers the concentration of AlCl₃ therein, there is a needto increase the activity of Al in the unconverted portion of stream SP3Ato deposit the desired amount of Al on the substrates S. This isachieved by passing the unconverted portion of the gas stream SP3Athrough an Al—Cr or Al—Co particulates bed B6 located downstream fromthe yttrium bed B4 in reaction chamber 94 as shown in FIG. 3. The Alalloy particulates can be comprised of 44 weight % Cr and balance Al andhave a size in the range of greater than ¼ inch to less than ⅝ inch. Thetemperature of the bed B6 is generally the same as that of bed B4.

[0060] Passage of the unconverted portion of stream SP3A through the bedB6 of Al—Cr particulates converts aluminum trichloride to aluminumsubchlorides that have increased activity compared to the trichloride,resulting in greater deposition of Al on the heated substrates S. Thealuminum subchloride gas component S3B and the YCl₃ component S3Agenerated inside the retort R flow in contact with the heated substratesS located thereabove to codeposit Al and Y thereon and form an aluminidediffusion coating including Y distributed uniformly therein.

[0061] An alternative coating apparatus to that of FIG. 3 is shown inFIG. 4. In FIG. 4, a coating gas stream S1 (described above with respectto FIG. 1) comprising aluminum trichloride in hydrogen carrier gas isintroduced into the retort R downstream of the yttrium bed B4.Introduction of the stream S1 downstream (above) of the Y bed B4 avoidsconversion of AlCl₃ therein to YCl₃. Thus, the need for the bed B6 ofAl—Cr particulates is eliminated. The stream SP3A described above withrespect to FIG. 3 is supplied to the retort R to generate the necessaryYCl₃ in generator 92 inside the retort R. The stream S1 and SP3A withYCl₃ generated inside the retort R constitute coating gas stream S3′that is flowed in contact with the heated substrates S locatedthereabove to codeposit Al and Y thereon and form an aluminide diffusioncoating including Y distributed uniformly at a particular coating regionor throughout the coating.

[0062] Because of the pyrophoric nature of yttrium hydride, it isnecessary to slowly oxidize the yttrium bed B4 with 1 volume % CO₂ in aninert carrier gas stream (not shown) before the CVD reactor or retort Ris opened at the end of a coating run.

[0063] Aluminide coatings typically having 20 to 25 weight % Al andhaving a trace of Y distributed therein as determined by electronmicroprobe analysis have been produced on PWA 1480, Rene 41 and IN738substrates after 10 to 20 hours at 1080° C. in the retort R andexhibited cyclic oxidation resistance in the aforementioned oxidationtest about 2 to 5 times better than simple aluminide coatings.

[0064] Referring to FIG. 5, coating apparatus/method for forming analuminide coating including two reactive elements are shown. Inparticular, for purposes of illustration and not limitation, coatingapparatus/method are shown to produce a CVD aluminide diffusion coatingincluding Hf and Y on substrates S.

[0065] In FIG. 5, the aforementioned stream S2 described above withrespect to FIG. 1 and comprising HfCl₄ in a He carrier gas is introducedinto the retort R via inlet I. Also, the stream SP3A described abovewith respect to FIGS. 3 and 4 and comprising AlCl₃ in a He/hydrogencarrier gas is introduced concurrently into the retort R via inlet I.The streams flow through the yttrium bed B4 and Al—Cr particulates bedB6 like those described with respect to FIG. 3.

[0066] In passing through the yttrium bed B4, a portion of the AlCl₃ instream SP3A is converted to YCl₃ as described above some HfCl₄ in streamS2 is also converted to YCl₃ by passage through the yttrium bed B4 as aresult of the relative free energies of formation shown in the Tablebelow: TABLE FREE ENERGY OF FORMATION REACTION PER MOLE OF Cl₂ 2/3 Y +Cl₂ = 2/3 YCl₃ −112,520 cal/mole 1/2 Hf + Cl₂ = 1/2 HfCl₄ −100,770cal/mole 2/3 Al + Cl₂ = 2/3 AlCl₃ −93,818 cal/mole 1/2 Zr + Cl₂ = 1/2ZrCl₄ −84,397 cal/mole Cr + Cl₂ = CrCl₂ −58,560 cal/mole 1/2 Si + Cl₂ =1/2 SiCl₄ −56,705 cal/mole

[0067] As can be seen, YCl₃ is more thermodynamically stable than AlCl₃so that a portion of the latter in stream SP3A is converted to YCl₃during flow through the yttrium bed B4. However, YCl₃ is only slightlymore stable than HfCl₄. As a result, a majority of the HfCl₄ in streamS2 passes through the bed B4 without conversion to YCl₃.

[0068] Since a portion of the stream SP3A is converted to YCl₃, theunconverted portion thereof is passed through the Al—Cr particulates bedB6 in order to increase the activity of Al in the manner described abovewith respect to FIG. 3; i.e. by forming aluminum subchlorides(AlCl_(x)). The HfCl₄ plus the AlCl_(x) and YCl₃ gas componentsgenerated inside the retort R flow in contact with the heated substratesS located thereabove to codeposit Al, Hf and Y thereon and form analuminide diffusion coating including Hf and Y distributed uniformly ata particular waiting region or throughout the coating for improvedoxidation resistance. An aluminide coating was produced on PWA 1480, PWA1484, Rene 80 and IN 738 substrates using coating apparatus/method ofFIG. 5 (13 hours at 1080° C. in retort R) to include 23 weight % Al,0.22 weight % Hf and 0.015 weight % Y as determined by electronmicroprobe analysis.

[0069] The coating apparatus of FIG. 5 has been used to form aluminidecoatings including Zr and Y even though ZrCl₄ is slightly less stablethan AlCl₃ from the Table set forth above. As a result of this reducedstability, a large fraction of the ZrCl₄ which enters the yttrium bed B4is converted to YCl₃. Nonetheless, Al, Zr, and Y can be codeposited onthe heated substrates S using the apparatus of FIG. 5, although it ismore difficult to predict the Zr concentration in the aluminide coatingas compared to the Hf concentration using the same apparatus. Analuminide coating was produced on PWA 1480, PWA 1484, PWA 663, Rene 41,Rene 142, IN 713 and IN 738 substrates using coating apparatus/method ofFIG. 5 (13 hours at 1080° C. in retort R) to include 0.18 weight % Zrand 0.265 weight % Y in one trial and 0.11 weight Zr and 0.32 weight % Yin another trial with Al typically being about 23 weight % as determinedby electron microprobe analysis.

[0070] The aforementioned CVD aluminide coatings including Hf plus Y andHf plus Zr and Y exhibited a cyclic oxidation resistance about 2 to 5times better than simple aluminide coatings in the aforementionedoxidation test.

[0071] Referring to FIG. 6, coating apparatus/method are shown forforming a CVD aluminide coating including Si and Y on substrates S. Inparticular, the stability of SiCl₄ is so much lower than that of YCl₃that nearly all of the SiCl₄ which enters the yttrium bed B4 isconverted to YCl₃. As a result, the aforementioned stream S1′ describedabove with respect to FIG. 2 and comprising cogenerated AlCl₃ and SiCl₄in hydrogen carrier gas is introduced into the retort R above(downstream) of the yttrium bed B4 so as not to react therewith. Thestream SP3A described above with respect to FIGS. 3 and 4 and comprisingAlCl₃ in a He/hydrogen carrier gas is introduced into the retort R belowthe yttrium bed B4 so as to flow through the bed B4 where a portion ofthe AlCl₃ is converted to YCl₃ as described with respect to FIGS. 3 and4. In this way, Al, Si, and Y are continuously codeposited on the heatedsubstrates S to form an aluminide coating including Si and Y uniformlydistributed therein. An aluminide coating was produced on PWA 1480, PWA1484, Rene 41, IN 738, IN 792, Mar-M 247, and GTD-111 substrates usingcoating apparatus/method of FIG. 6 (4 hours at 1080° C. in retort R) toinclude less than 1 weight % Si and a trace of % Y with 24 weight % Alas determined by electron microprobe analysis.

[0072] The coating gas streams illustrated in FIG. 6 are effective toprovide continuous codeposition of aluminum, silicon and yttrium. Sincesilicon and yttrium form stable intermetallic compounds, it is necessaryto provide such continuous deposition on the substrate of silicon toobtain a uniform distribution of silicon and yttrium at a particularregion of or throughout the aluminide coating.

[0073] For purposes of illustration only, the aforementioned coating gasstream S1′ and coating gas stream SP3A were employed for contacting thenickel base superalloy substrates heated to 1080° C. in the CVD retort60. The flow rate of the coating gas stream S1′ was 3 scfh and the flowrate of precursor halide gas stream SP3A was 7 scfh. An aluminidediffusion coating was formed on the substrates after 4 hours andcomprised 23 weight % Al, trace levels of Y, less than 1 weight % Si andbalance Ni.

[0074] CVD aluminide diffusion coatings comprising other compositionshave been produced by the invention. For example, CVD nickel aluminidecoatings have been produced on Ni base superalloy substrates to include0.03 weight % Si and 0.1 weight % Y.

[0075] CVD aluminide coatings including combinations of Si and one ormore of hafnium, zirconium, yttrium, and other gettering elements can beformed by using the apparatus of FIGS. 5 and 6 and combinations thereof.For example, an aluminide coating including Si, Hf or Zr, and Y ascoating alloying elements can be formed using the apparatus of FIG. 6wherein the aforementioned coating gas streams are introduced and/orgenerated in the retort R.

[0076] Thus, the invention provides apparatus and method for continuousCVD codeposition of Al and one or more reactive elements on superalloyand other substrates. The substrates may be treated to include Pt, Pdand other metallic layers or enriched surface zones pursuant toparticular coating processes used in the art. The reactive elements canform intermetallic compounds with Si in a uniform distribution at aparticular region of or throughout the coating to provide improvedcoating oxidation resistance.

[0077] The present invention thus provides a coated substrate comprisinga superalloy or other substrate and a chemically vapor depositedaluminide diffusion coating thereon having a uniform dispersion ordistribution of one or more reactive elements therein at a particularcoating region or throughout the coating.

[0078] Although the invention has been described in terms of certainspecific embodiments, it is understood that modifications and changescan be made thereto within the scope of the invention and appendedclaims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of forming bychemical vapor deposition on a substrate an aluminide coating includinga reactive element for a surface active substrate impurity element,comprising flowing a first halide precursor gas in a carrier gas incontact with a first source comprising aluminum disposed outside acoating retort to generate an aluminum halide first coating gas, flowinga second halide precursor gas in an inert carrier gas in contact with asecond source comprising the reactive element disposed outside thecoating retort to generate a second halide coating gas of said getteringelement, and introducing the first and second coating gases concurrentlyinto a coating retort in which the substrate at elevated temperature isdisposed.
 2. The method of claim 1 wherein the first coating gas isformed by flowing hydrogen chloride in a hydrogen carrier gas in contactwith Al particulates heated to a reaction temperature to form aluminumtrichloride.
 3. The method of claim 1 wherein the second coating gas isformed by flowing hydrogen chloride in an inert carrier gas in contactwith particulates selected from the group consisting essentially of Hfand Zr heated to a reaction temperature to form the tetratrichloridethereof.
 4. The method of claim 1 wherein the first coating gas isflowed in contact with said first source at a flow rate controlled togenerate an aluminum halide coating gas while leaving a portion of thehalide precursor gas unreacted and then flowing the unreacted portion ofthe halide precursor gas in contact with a second source comprisingsilicon to cogenerate a silicon halide coating gas from said halideprecursor gas.
 5. The method of claim 4 wherein the halide precursor gasis flowed through a generator reaction chamber having the source ofaluminum therein proximate an inlet region of the chamber and the sourceof silicon therein proximate an outlet region of the chamber such thatthe halide precursor gas first contacts the source of aluminum and thencontacts the source of silicon to generate a coating gas streamcomprising aluminum halide and silicon halide.
 6. The method of claim 4wherein the halide precursor gas is flowed through a first generatorreaction chamber having the source of aluminum therein and then througha second generator upstream of said first generator and having thesource of silicon therein such that the halide precursor gas firstcontacts the source of aluminum and then contacts the source of siliconto generate a coating gas stream comprising aluminum halide and siliconhalide.
 7. The method of claim 1 wherein the second halide coating gasis flowed in contact with said second source at a flow rate controlledto generate a hafnium halide coating gas while leaving a portion of thehalide precursor gas unreacted and then flowing the unreacted portion ofthe halide precursor gas in contact with a second source comprisingzirconium to cogenerate a zirconium halide coating gas from said halideprecursor gas.
 8. The method of claim 7 wherein the halide precursor gasis flowed through a generator reaction chamber having the source ofhafnium therein proximate an inlet region of the chamber and the sourceof zirconium therein proximate an outlet region of the chamber such thatthe halide precursor gas first contacts the source of hafnium and thencontacts the source of zirconium to generate a coating gas streamcomprising hafnium halide and zirconium halide.
 9. The method of claim 7wherein the halide precursor gas is flowed through a first generatorreaction chamber having the source of hafnium therein and then through asecond generator upstream of said first generator and having the sourceof zirconium therein such that the halide precursor gas first contactsthe source of hafnium and then contacts the source of zirconium togenerate a coating gas stream comprising hafnium halide and zirconiumhalide.
 10. A method of chemical vapor deposition on a substrate offirst and second elements, comprising flowing a halide precursor gas ina carrier gas in contact with a first source comprising a first elementdisposed outside a coating retort to generate a first halide coatinggas, flowing the first coating gas into the coating retort in contactwith a second source comprising a second element disposed inside thecoating retort to convert a portion of the first coating gas to a halidecoating gas of the second element, and contacting the first and secondcoating gases concurrently with the substrate at elevated temperature inthe coating retort to codeposit the first and second elements on thesubstrate.
 11. A method of forming by chemical vapor deposition on asubstrate an aluminide coating including a reactive element for asurface active substrate impurity element, comprising flowing a halideprecursor gas in a carrier gas in contact with a first source comprisingaluminum disposed outside a coating retort to generate an aluminumhalide coating gas, flowing the first coating gas into the coatingretort in contact with a second source comprising the reactive elementdisposed inside the coating retort to convert a portion of the firstcoating gas to a halide coating gas of the reactive element, andcontacting the coating gases concurrently with the substrate at elevatedtemperature in the coating retort to form an aluminide coating includingthe reactive element.
 12. The method of claim 11 wherein the source ofthe reactive element inside the coating retort comprises yttriumparticulates.
 13. A method of forming by chemical vapor deposition on asubstrate an aluminide coating including a reactive element for asurface active substrate impurity element, comprising flowing a halideprecursor gas in a carrier gas in contact with a first source comprisingaluminum disposed outside a coating retort to generate an aluminumhalide coating gas, flowing the first coating gas into the coatingretort in contact with a second source comprising the reactive elementdisposed inside the coating retort to convert a portion of the firstcoating gas to a halide coating gas of the reactive element, flowing theunconverted portion of said first coating gas in contact with asecondary source comprising an aluminum alloy in the coating retort toincrease the activity of aluminum therein, and contacting the coatinggases concurrently with the substrate at elevated temperature in thecoating retort to form an aluminide coating including the reactiveelement.
 14. The method of claim 13 wherein the source of the reactiveelement inside the coating retort comprises yttrium particulates. 15.The method of claim 13 wherein the first coating gas comprises aluminumtrichloride and wherein the activity of aluminum in the unconvertedportion of the first coating gas is increased by forming subchlorides ofaluminum trichloride by contact with said secondary source.
 16. Themethod of claim 15 wherein the secondary source comprises an Al—Cr orAl—Co alloy.
 17. The method of claim 13 wherein the first coating gascomprises aluminum trichloride and a tetrachloride of the reactiveelement formed outside the coating retort.
 18. The method of claim 17wherein the first coating gas includes a tetrachloride of hafnium orzirconium formed outside the coating retort to form an aluminide coatingincluding two or more reactive elements.
 19. The method of claim 17further including introducing a silicon halide coating gas in thecoating retort in a manner to by-pass the second source and concurrentlycontact the substrate with said coating gases to form an aluminidecoating including Si and the reactive element.
 20. A coated substratecomprising a superalloy substrate and a chemically vapor depositedaluminide diffusion coating thereon including at a coating region orthroughout the coating a dispersion of a reactive element for a surfaceactive substrate impurity element by virtue of said reactive elementbeing codeposited with aluminum on said substrate.
 21. The coatedsubstrate of claim 20 wherein the reactive element is selected from thegroup consisting of Hf, Zr, Si and Y.
 22. The coated substrate of claim21 wherein two or more reactive elements selected from the groupconsisting essentially of Hf, Zr, Si, and Y are dispersed in the coatingby virtue of codeposition with aluminum on the substrate.
 23. Apparatusfor forming a chemical vapor deposition coating gas, comprising meansfor providing a halide precursor gas, means for flowing the halideprecursor gas in contact with a first source comprising metal, coatingretort means, and means for flowing the first coating gas into thecoating retort in contact with a source of a reactive element inside theretort to convert a portion of the first coating gas to a halide coatinggas of the gettering element that contacts a substrate in the retortwith the first coating gas to codeposit the metal and the reactiveelement on the substrate.
 24. The apparatus of claim 23 including asecondary source of the metal upstream of the reactive element sourcefor contacting the unconverted portion of the first coating gas.
 25. Theapparatus of claim 24 including means for introducing another metalhalide coating gas into the retort upstream of the gettering elementsource to codeposit said another metal along with said metal and thereactive element on the substrate.